What an anti-virus program does.

Let's pretend that your computer is a treehouse club that you like to play in and invite friends over to.

Anti-Virus is like a bouncer standing at the door of the club. Except this guy has a list of bad people who he looks out for and when they try to go into the club he stops them.
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f there are new bad kids in the neighborhood that the bouncer doesn't know about he might let them in so it is important to make sure his list of bad people is kept up-to-date.

It is also important for you to avoid inviting random kids you meet in bad areas of town to come and play. Some places are known hangouts for trouble makers.

Norton Anti Virus is a big heavy slow bouncer who asks for money every year and he is clumsy and gets in the way a lot when you are trying to play in the treehouse.

Microsoft Security Essentials is a skinny fast martial arts expert who stays out of sight except when he is needed and has a good up-to-date list of bad kids all the time without asking you for any money.

There are other good free bouncers out there too. I happen to like Microsoft Security Essentials at the moment. A few years ago AVG Free was good but he has put on a few pounds and slowed down lately in my opinion.

I like to have Malwarebytes Anti Malware as well. He is a bouncer who is extra good at recognizing kids who try to come in and spy on you and pass the information on to their friends. He stops kids listening near the door for your secret passwords and any gossip about you and your friends.

Even the best bouncers cannot help you if you invite people and show them in yourself. Some bad kids pretend to be bouncers and trick you into letting them into the treehouse. Don't fall for this!